1. Field of the Invention:
The present invention relates to a membrane unit, an apparatus comprising a membrane unit and a method for cleaning blood by removing metabolite contaminants, i.e., toxic metabolites and metabolites normally contained in urine, from blood.
2. Description of the Prior Art:
Diafiltration and hemoperfusion are known methods for removing metabolites from blood. Metabolites are those components of the living cells, which control the normal course of metabolic reactions, as well as products of metabolism formed or catabolized in human or animal organisms, such as urea, creatinine, peptides, carbohydrates and electrolytes, e.g., sodium or potassium salts and water. In diafiltration, separation is achieved by means of filtration via selectively permeable membranes hereinafter referred to as permselective membranes. Hemoperfusion is based on the principle of adsorption.
In the diafiltration process the driving force is an adjustable pressure gradient, which determines the transport rate. Any substance having molecules of a size below the porosity limit of the permselective membrane is pressed out as an ultrafiltrate in the same ratio of concentration as in blood. The ultrafiltrate may be rejected; however, a certain portion of the ultrafiltrate extracted from the blood must be returned to the blood stream with all vital substances in a physiological ratio of concentration. Like hemodialysis, diafiltration is primarily used for treating persons suffering from chronic kidney diseases.
Conversely, hemoperfusion is based on a different principle of operation, and its application has been up to now almost exclusively limited to those cases in which a particularly rapid detoxification of the blood is required, as in acute failures of the liver or intoxications. In this method, adsorbents, such as activated carbon or macroporous resins are used to adsorb toxic metabolites. The adsorbents, which are usually enveloped by a porous membrane material, are generally used in granulated form, enclosed as microcapsules in an aqueous suspension, coated upon support webs or used as fiber bundles disposed in columns through which contaminated blood passes. The enveloping of the adsorbents in a porous membrane material is preferred since it prevents direct contact with the blood which improves blood compatibility. Nevertheless, there is a considerable risk of damaging the blood, particularly from loss of blood cells and proteins, from microembolisms due to washed-out adsorbent particles, and from an interruption of the steady flow in the column passed by the blood. In view of this high risk, the use of hemoperfusion is limited to cases in which the patient is comatose.
An additional disadvantage of the hemoperfusion system is that the adsorbent is not adequately capable of adsorbing all metabolites which must be removed from the blood, in particular metabolites normally contained in urine, such as water, urea, electrolytes and ammonia. Even the use of additional complex and expensive measures, such as the use of enzymes, such as urease, in microcapsules, do not result in a simple and satisfactory removal of these metabolites.
It has heretofore been suggested to connect diafiltration and hemoperfusion devices in series in order to utilize the rapid detoxifying action of hemoperfusion and to remove the non-adsorbable metabolites normally contained in urine. This procedure is, however, disadvantageous due to the high blood-filling and residual volume of the devices; in particular, the patient is exposed to the risk of hypotension. Moreover, in addition to the problems inherent in the use of the hemoperfusion device previously described, there is the danger of damaging the blood due to the use of additional, complex apparatus. Finally, it is difficult to coordinate the devices, and further their operation is extremely expensive.
It has also been proposed to lead the ultrafiltrate from a diafiltration device into a device containing toxin-adsorbing substances and subsequently return the detoxified filtrate to the blood stream. In this process heavy metals such as copper are removed by activated carbon and hydrated zirconium oxide. However, such devices operate stepwise and thus have the same disadvantages as a series connection of the devices, particularly due to the high residual volume of the apparatus. Thus, there remains a need in the art for a simple but efficient means of removing metabolite contaminants from blood that avoids the dangers and problems associated with existing systems.